The proposed studies focus on the reaction of the mammalian uterus to an implanting blastocyst and early events in placental development. Rabbits will be used as the experimental model and we will attempt to define the respective roles of blastocyst and endometrial tissues in the establishment of pregnancy. In particular, we will be interested in (1) the sequence of vascular events involving prostaglandins which are correlated with the uterine decidual response and the possible interaction of prostanoic compounds with other vasoactive agents (e.g., histamine, serotonin) at the uterine level, (2) characterizing the structural changes associated with blastocyst-endometrial interaction and the subsequent development of the maternal placenta (decidua) and, (3) determining the identity and biological significance of trophoblastic crystalline inclusions in early blastocysts.